11 Module Types and Signatures

OCaml has a similar feature to interfaces called signatures

module type Fact = sig
	(** [fact n] is n factorial. *)
	val fact : int -> int
end

module type Fact = sig val fact : int -> int end

Here we say "module type," because we specify what type a module has, rather than it's value. sig represents a signature. Here we specify that there must be a value called "fact," that takes an int and returns an int. It's the same type of signature that utop returns!

module RecursiveFact : Fact = struct
	let rec fact n =
		if n = 0 then 1 else n * fact (n - 1)
end

module RecursiveFact : Fact

Here, we implement RecursiveFact which is of type Fact. The typechecker makes sure that the types are correct.

module ImNotFact : Fact = struct
	let printHello = print_endline "Hello"
end

File "[3]", lines 1-3, characters 26-3:
1 | ..........................struct
2 | 	let printHello = print_endline "Hello"
3 | end
Error: Signature mismatch:
       Modules do not match:
         sig val printHello : unit end
       is not included in
         Fact
       The value `fact' is required but not provided
       File "[1]", line 3, characters 1-22: Expected declaration

But you can have extra values!

module FactWithAddOne : Fact = struct
	let fact = RecursiveFact.fact
	let add_one = ( + ) 1
end

module FactWithAddOne : Fact

FactWithAddOne.add_one 1

File "[5]", line 1, characters 0-22:
1 | FactWithAddOne.add_one 1
    ^^^^^^^^^^^^^^^^^^^^^^
Error: Unbound value FactWithAddOne.add_one

You can have multiple modules that implement the same signature:

module TailRecursiveFact : Fact = struct
	let rec fact_aux acc n = 
		if n = 0 then acc
		else fact_aux (n * acc) (n - 1)
	
	let fact = fact_aux 0
end

module TailRecursiveFact : Fact